Fix BackgroundCompositor not retaining input metadata

[djhoese]

While working on #854 I noticed that sensor information wasn't being handled the way I wanted by the BackgroundCompositor. I then discovered that none of the metadata is being handled. This PR attempts to fix that and cleans up a few other things that flake8/pre-commit were complaining about.

This includes removing the use of some deprecated functions.

This might fix #902 (FYI @peters77)

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented

[peters77]

Thanks for the info. #902 seems not to be fixed by your modification, here my output:

  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 2657, in get_loc
    return self._engine.get_loc(key)
  File "pandas/_libs/index.pyx", line 108, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 1601, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 1608, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'L'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "make_msg3_germ_day.py", line 207, in <module>
    new_scene = global_scene.resample(area, radius_of_influence = 20000, nprocs=4)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/scene.py", line 1103, in resample
    keepables = new_scn.generate_composites()
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/scene.py", line 884, in generate_composites
    return self._read_composites(nodes)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/scene.py", line 858, in _read_composites
    self._generate_composite(item, keepables)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/scene.py", line 833, in _generate_composite
    **self.attrs)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/satpy/composites/__init__.py", line 1520, in __call__
    fg_band = foreground.sel(bands=band)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/xarray/core/dataarray.py", line 857, in sel
    **indexers_kwargs)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/xarray/core/dataset.py", line 1722, in sel
    self, indexers=indexers, method=method, tolerance=tolerance)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/xarray/core/coordinates.py", line 317, in remap_label_indexers
    obj, v_indexers, method=method, tolerance=tolerance
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/xarray/core/indexing.py", line 252, in remap_label_indexers
    dim, method, tolerance)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/xarray/core/indexing.py", line 181, in convert_label_indexer
    indexer = get_loc(index, label.item(), method, tolerance)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/xarray/core/indexing.py", line 108, in get_loc
    return index.get_loc(label, **kwargs)
  File "/home/cpeters/anaconda2/envs/pytroll/lib/python3.6/site-packages/pandas/core/indexes/base.py", line 2659, in get_loc
    return self._engine.get_loc(self._maybe_cast_indexer(key))
  File "pandas/_libs/index.pyx", line 108, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc
  File "pandas/_libs/hashtable_class_helper.pxi", line 1601, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas/_libs/hashtable_class_helper.pxi", line 1608, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'L' ```

[pnuu]

Looks good to me. Unfortunately can't test within the couple next days.

[pnuu]

Should've first checked the test results, but the fix seems to be pretty trivial :-)

[djhoese]

@peters77 If you try this branch again it should fix your issue.

[coveralls]

Coverage decreased (-0.01%) to 85.348% when pulling 5e92278 on djhoese:bugfix-background-compositor into d09edbc on pytroll:master.

[codecov]

Codecov Report

Merging #917 into master will decrease coverage by 0.01%.
The diff coverage is 88.67%.

@@            Coverage Diff             @@
##           master     #917      +/-   ##
==========================================
- Coverage   85.36%   85.35%   -0.02%     
==========================================
  Files         174      174              
  Lines       26023    26037      +14     
==========================================
+ Hits        22215    22223       +8     
- Misses       3808     3814       +6

Impacted Files	Coverage Δ
satpy/tests/compositor_tests/__init__.py	99.54% <100%> (ø)	⬆️
satpy/composites/__init__.py	75.09% <72.72%> (-0.26%)	⬇️
satpy/scene.py	90.47% <0%> (-0.18%)	⬇️

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[peters77]

@djhoese issue fixed if I use this branch. 👍 Now if #830 is fixed I can use a static day background image too.

[mraspaud]

Looks good to me, thanks for taking the time to sort things out!